XXhash is only fast with digest workloads, not hash tables. There the hash function needs to be fast and small, XXhash is too big and trashes the icache.
The benchmarks on the xxHash Github page were done on a 32-bit system using an old Visual C++ compiler. Of course you can beat hash functions that were explicitly designed for modern 64-bit architectures. The Core 2 lacks the CRC instruction introduced by the SSE 4.2 extension, which is the main reason Google wrote their hashes.
All modern hashes are designed for 64-bit architectures and some even rely on recent instruction sets. So a benchmark on an old 32-bit CPU isn't indicative of the performance you'll see on a modern processor, e.g. Metrohash will definitely not be 5x slower than xxHash.
I'm asking for an apples-to-apples comparison here. Using one figure from a random CPU on github and comparing it to another figure from a different random CPU on github is not an apples to apples comparison.
I don't think anyone claimed xxhash would be 5x faster than metrohash. (Maybe you are getting the 5x number from metrohash's claim that it is 5x faster than siphash?)
